Abstract
The formation of a cavitation bubble cloud due to the high intensity focused ultrasound (HIFU) with a laser-induced bubble near the focus has been investigated experimentally. The backscattering of HIFU having the nonlinear pressure waveform from the interface of a laser-induced bubble forms stronger tensile pressure field near the interface, which leads to the cavitation inception and the following cavitation bubble cloud formation. In the present study, the optical observation by the high-speed video camera with a frame rate up to 1.25 Mfps and the pressure measurement by the fiber-optic prove hydrophone (FOPH) are carried out simultaneously to analyse the cavitation bubble cloud formation. The pressure field with the stronger negative pressure is formed by the backscattering of HIFU, and the minimum negative pressure is measured at a distance of 0.3 times of the wavelength of ultrasound from the interface of a laser-induced bubble. It is also found that the cavitation bubble cloud grows conically along with the propagation axis of the incident wave. The cloud is composed of some layers of small bubbles and these layers appear stepwise in time. Thus, the growth of the cavitation bubble cloud is accompanied by multiple sudden increments of bubbles on the axial dimension leading to the multilayer formation of cavitation bubbles. The measurement by the FOPH reveals that when a laser-induced bubble exists, a stronger negative pressure than the incident wave is measured due to the backscattering of the incident ultrasound from the bubble interface. Negative peaks in the pressure waveform have a long-term fluctuation, and are correlated with the growth of the bubble cloud. The minimum negative pressure measured in the vicinity of the borderline for the growth of the bubble cloud is considered to be the pressure threshold of the cavitation inception, and is −24 MPa in the present experiment.
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